The present invention relates to systems and methods for engine and/or vehicle speed control for direct injection spark ignition engine applications.
Engine/vehicle speed control systems and methods attempt to achieve a relatively constant engine/vehicle speed corresponding to a driver or system desired speed, such as a vehicle speed selected for cruise control or an engine speed selected for idle. Conventional engine/vehicle speed controls are limited to a particular engine operating range and employ different strategies (algorithms or xe2x80x9ccontrollersxe2x80x9d) for idle speed control and cruise control, i.e. standard idle speed control methodology can not be extended to cruise control.
Airflow-based idle speed control strategies use an airflow actuator, such as an idle air bypass valve and/or electronically controlled throttle valve, as the primary actuator to control engine speed to a desired idle speed selected according to current operating conditions. Torque-based idle speed control strategies typically use ignition timing (spark) as the primary fast torque actuator to control engine speed by modifying engine brake torque. Spark is typically preferred for speed control because the engine output torque and resulting engine speed respond very quickly to changes in spark timing. Furthermore, spark has a broad range of authority over output torque, i.e. spark can be used to rapidly vary output torque to nearly zero, if desired. However, any departure of spark timing from the spark timing that gives maximum torque for a given amount of air and fuel (MBT) may result in a fuel economy penalty.
Speed control strategies for gasoline engines, whether using spark, air, or fuel, require consideration of the effect on emissions and associated emission control devices. For conventional engines, where three-way catalysts are used, the air/fuel ratio must be tightly controlled near the stoichiometric ratio to prevent premature reduction of the catalyst conversion efficiency. Likewise, proper emissions control is more easily achieved by maintaining appropriate catalyst operating temperatures.
In a lean burn direct injected spark ignited (DISI) engine (which includes direct injection stratified charge or DISC engines), the air/fuel ratio is not constrained to a narrow range about the stoichiometric value. These engines can operate in a stratified or homogeneous mode with improved fuel economy due primarily to reduced pumping losses during stratified mode operation, particularly at low engine speeds and loads such as experienced at idle. While operation in the homogeneous mode may employ similar control strategies as a conventional port fuel-injected engine, stratified mode operation is more complex and provides new opportunities for improving speed control at idle and during cruise control.
An object of the present invention is to provide systems and methods for engine/vehicle speed control during stratified mode operation of DISI engines, particularly lean burn DISC engines.
In carrying out the above object and other objects, advantages, and features of the present invention, one embodiment includes a system and method for controlling engine/vehicle speed in a direct injection spark ignition internal combustion engine application. The system/method determine a speed error based on a difference between an actual speed and a scheduled speed, determine a desired engine torque based on the speed error, determine required fuel to deliver the desired engine torque, calculate an air/fuel ratio based on the required fuel and current airflow, and control at least one fuel injector to deliver the required fuel if the air/fuel ratio is within an acceptable air/fuel ratio range for current engine operating conditions.
Methods and systems for engine/vehicle speed control according to the present invention may be used in DISI engines having variable valve timing or variable cam timing, in addition to any throttled or unthrottled DISI engines, including lean burn DISI engines.
The present invention provides a number of advantages over prior art control strategies. For example, the present invention uses fuel as the primary fast torque actuator rather than spark because engine operation is not limited to a narrow range of stoichiometry. Spark can then be maintained substantially at MBT to enhance fuel economy. The present invention provides engine/vehicle speed regulation while maintaining engine set-points to reduce performance penalties due to compromises in control to achieve both objectives. When air/fuel ratio limits prohibit the control of torque using fuel, the present invention performs speed regulation only and uses airflow control as the torque actuator for this task. Airflow control can be provided by an electronically controlled throttle, or using variable valve timing or variable cam timing. The present invention uses a two-degree of freedom controller design to respond differently to errors arising from load torques suddenly applied to the engine than to speed set-point command changes. The present invention provides systems and methods for speed control which function properly over the entire operating region of the engine such that they can be used for both idle speed control and cruise control.
The above advantages and other advantages, objects, and features of the present invention, will be readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.